A tooth crown morphology framework for interpreting the diversity of primate dentitions

Variation in tooth crown morphology plays a crucial role in species diagnoses, phylogenetic inference, and the reconstruction of the evolutionary history of the primate clade. While a growing number of studies have identified developmental mechanisms linked to tooth size and cusp patterning in mammalian crown morphology, it is unclear (1) to what degree these are applicable across primates and (2) which additional developmental mechanisms should be recognized as playing important roles in odontogenesis. From detailed observations of lower molar enamel-dentine junction morphology from taxa representing the major primate clades, we outline multiple phylogenetic and developmental components responsible for crown patterning, and formulate a tooth crown morphology framework for the holistic interpretation of primate crown morphology. We suggest that adopting this framework is crucial for the characterization of tooth morphology in studies of dental development, discrete trait analysis, and systematics.

Primate tooth crown nomenclature revisited

Cusp patterning on living and extinct primate molar teeth plays a crucial role in species diagnoses, phylogenetic inference, and the reconstruction of the evolutionary history of the primate clade. These studies rely on a system of nomenclature that can accurately identify and distinguish between the various structures of the crown surface. However, studies at the enamel-dentine junction (EDJ) of some primate taxa have demonstrated a greater degree of cusp variation and expression at the crown surface than current systems of nomenclature allow. In this study, we review the current nomenclature and its applicability across all the major primate clades based on investigations of mandibular crown morphology at the enamel-dentine junction revealed through microtomography. From these observations, we reveal numerous new patterns of lower molar accessory cusp expression in primates. We highlight numerous discrepancies between the expected patterns of variation inferred from the current academic literature, and the new patterns of expected variation seen in this study. Based on the current issues associated with the crown nomenclature, and an incomplete understanding of the precise developmental processes associated with each individual crown feature, we introduce these structures within a conservative, non-homologous naming scheme that focuses on simple location-based categorisations. Until there is a better insight into the developmental and phylogenetic origin of these crown features, these categorisations are the most practical way of addressing these structures. Until then, we also suggest the cautious use of accessory cusps for studies of taxonomy and phylogeny.

Neutral evolution of human enamel-dentine junction morphology

Teeth have been studied for decades and continue to reveal information relevant to human evolution. Studies have shown that many traits of the outer enamel surface evolve neutrally and can be used to infer human population structure. However, many of these traits are unavailable in archaeological and fossil individuals due to processes of wear and taphonomy. Enamel-dentine junction (EDJ) morphology, the shape of the junction between the enamel and the dentine within a tooth, captures important information about tooth development and vertebrate evolution and is informative because it is subject to less wear and thus preserves more anatomy in worn or damaged specimens, particularly in mammals with relatively thick enamel like hominids. This study looks at the molar EDJ across a large sample of human populations. We assessed EDJ morphological variation in a sample of late Holocene modern humans (n = 161) from archaeological populations using μ-CT biomedical imaging and geometric morphometric analyses. Global variation in human EDJ morphology was compared to the statistical expectations of neutral evolution and "Out of Africa" dispersal modeling of trait evolution. Significant correlations between phenetic variation and neutral genetic variation indicate that EDJ morphology has evolved neutrally in humans. While EDJ morphology reflects population history, its global distribution does not follow expectations of the Out of Africa dispersal model. This study increases our knowledge of human dental variation and contributes to our understanding of dental development more broadly, with important applications to the investigation of population history and human genetic structure.

First record of a fossil monkey (Primates, Cercopithecidae) from the Late Pliocene of Serbia

The cercopithecid fossil record of the Balkan Peninsula extends from the Late Miocene to the Early Pleistocene, but to date no fossils of non-human primates have been identified in Serbia. Here we report the identification of two primate teeth from Ridjake, a rich paleontological site in western Serbia. NHMBEO 042501 is an upper third molar with heavy occlusal wear and taphonomic weathering. NHMBEO 042502 is a well-preserved lower third molar with only minor damage to the cusps and root apices. We performed an analysis of non-metric traits and made bivariate comparisons of crown linear measurements in order to assess the taxonomic affinity of the molars. Both show typical papionin occlusal patterns and relatively large overall sizes. In combination with the early Villafranchian (MN16) age of the site, we attribute both Ridjake primate fossils to cf. Paradolichopithecus sp. This represents the first identification of a non-human primate in Serbia, and the first identification of any primate in the Neogene period of Serbia. Along with recent hominin discoveries, the Ridjake fossils contribute to the growing primate fossil record in Serbia, and indicate the need for increased research into fossil primates in the country.

Identification of a derived dental trait in the papionini relative to other old world monkeys

Variation in the shape of teeth provides an immense amount of information about the evolutionary history and adaptive strategy of a mammalian lineage. Here, we explore variation in the expression of a purported molar lingual remnant (the interconulus) across the Old World Monkeys (Primates: Cercopithecidae) with the aim of elucidating a component of the adaptive radiation of this family. This radiation is characterized by a wide geographic range (Asia and Africa) as well as diverse dietary niches. While all of the cercopithecids are distinguished by their derived bilophodont molars, the colobines have evolved taller and more pointed cusps compared with the cercopithecines. We investigate whether the interconulus also correlates with phylogenetic affinity and/or dietary adaptation. We assess the frequency and range of interconulus expression in 522 specimens representing seven species of Old World Monkeys (Cercopithecus mitis, n = 78; Macaca fascicularis, n = 85; Macaca mulatta, n = 70; Papio hamadryas, n = 55; Colobus guereza, n = 76; Presbytis melalophos, n = 82; Presbytis rubicunda, n = 76). Results show that the interconulus has a significantly higher frequency and degree of expression in Tribe Papionini and exhibits ordered metameric variation with greatest expression in the third molars. Given the rarity of the interconulus in other closely related taxa, and its morphological distinction from the purportedly homologous features in other primates, we interpret the high degree of expression of the interconulus to be a trait derived in papionins that originated in the Miocene baboon/macaque ancestor.

Genetic integration of molar cusp size variation in baboons

Many studies of primate diversity and evolution rely on dental morphology for insight into diet, behavior, and phylogenetic relationships. Consequently, variation in molar cusp size has increasingly become a phenotype of interest. In 2007 we published a quantitative genetic analysis of mandibular molar cusp size variation in baboons. Those results provided more questions than answers, as the pattern of genetic integration did not fit predictions from odontogenesis. To follow up, we expanded our study to include data from the maxillary molar cusps. Here we report on these later analyses, as well as inter-arch comparisons with the mandibular data. We analyzed variation in two-dimensional maxillary molar cusp size using data collected from a captive pedigreed breeding colony of baboons, Papio hamadryas, housed at the Southwest National Primate Research Center. These analyses show that variation in maxillary molar cusp size is heritable and sexually dimorphic. We also estimated additive genetic correlations between cusps on the same crown, homologous cusps along the tooth row, and maxillary and mandibular cusps. The pattern for maxillary molars yields genetic correlations of one between the paracone-metacone and protocone-hypocone. Bivariate analyses of cuspal homologues on adjacent teeth yield correlations that are high or not significantly different from one. Between dental arcades, the nonoccluding cusps consistently yield high genetic correlations, especially the metaconid-paracone and metaconid-metacone. This pattern of genetic correlation does not immediately accord with the pattern of development and/or calcification, however these results do follow predictions that can be made from the evolutionary history of the tribosphenic molar.

Protostylid variation in Australopithecus

Recent advances in computed tomography (CT) and genetics provide new insights into the morphology and biology of anatomical traits, particularly in the dentition. As we move towards a fuller understanding of the genetic and developmental bases for dental traits, we need to reassess the taxonomic and evolutionary variation of established characters. Quantitative genetic analyses indicate that the degree of expression of upper and lower primate cingular remnants are genetically interdependent. This has serious evolutionary implications that need to be explored for fossil hominids. Studies of Carabelli's cusp, a cingular remnant on hominid upper molars, have been advanced through both genetic and CT analyses setting the stage for such an investigation. But its mandibular morphological homologue, the protostylid has not been similarly studied. This paper represents the first step towards a quantitative understanding of the variation and evolution of this trait in early hominids. Since the first discoveries of Australopithecus specimens in South Africa more than sixty years ago, cingular features on lower molars have played a significant role in the description and comparison of hominid taxa. This largely qualitative history is reviewed. Because the modern human classification system for protostylid variation does not adequately describe the variation seen in Australopithecus samples, a quantification scheme with six expression states is established. Using this new protocol, protostylid variation in six species of Australopithecus is assessed. Results from these analyses show that the distribution of the degree of protostylid expression in these species is highly varied. When first, second, and third molar samples are considered separately, the distribution of expression states is found to differ considerably within the same species. These results provide a foundation for further genetic and developmental research on the evolutionary history of the hominid dentition.

Genetic contributions to expression of the baboon cingular remnant

Primitive mammalian molar morphology is characterised in part by a ridge of enamel that encircles the entire base of the molar crown, the cingulum. Many higher primates have reduced the cingulum, but often retain remnant features on the lingual surface of maxillary molars and the labial surface of mandibular molars. Two of these remnants in cercopithecoid primates, the interconulus and interconulid, are morphologically similar though the interconulus is found on maxillary molars and the interconulid is located on mandibular molars. Here we present results from a quantitative genetic analysis of expression of these two traits in a sample of 479 modern savannah baboons from the Southwest Foundation for Biomedical Research (SFBR). We found that both traits are significantly heritable with little variance attributable to other factors, such as sex, age, and molar crown size. Bivariate analyses yielded point estimates for genetic correlations between left and right side expression that are either equal to or not significantly different from 1.0; meaning that 100% of their additive genetic variance is due to the effects of the same gene or suite of genes. By contrast, our estimates of the genetic correlations between maxillary and mandibular expression of this trait range from 0.52 to 0.72, suggesting that 28-52% of the additive genetic variance in the interconulus and interconulid is due to the effects of shared genes. These results demonstrate that intra-arch expression is characterised by complete pleiotropy whereas inter-arch expression is caused by incomplete pleiotropy. These results are relevant to dental developmental studies as well as paleontological analyses of the evolution of the primate dentition.